In a search for their ancestors, more than 140 people with variations of the last name Kincaid have taken DNA tests and shared their results on the Internet.
They have found war heroes, sailors and survivors of the Irish potato famine.
They have also stumbled upon bastards, liars and two-timers.
Much of it is ancient history, long-dead ancestors whose dalliances are part of the intrigue of amateur genealogy. But sometimes the findings strike closer to home.
In one case, two brothers were surprised to discover they had different fathers. They confronted their elderly mother, who denied the most obvious possibilities -- that she had been unfaithful to her husband, the man they had always known as Dad, or that one son was adopted.
"It has been traumatic for some to discover their true lineage through the DNA tests," said Don Kincaid, a 76-year-old Texan who oversees the Kincaid surname project and witnessed the brothers' ordeal.
As genetic testing becomes more widespread for medical information, forensics and ancestral research, more people are accidentally uncovering family secrets. Among the most painful are so-called "non-paternity events," cases in which Dad turns out to be someone else.
"It's going to be more and more of a problem," said Dr. Eric Topol, chief of genomic medicine at Scripps Research Institute in La Jolla. Increasing numbers of people will be asking their spouses and parents: "What happened 25 years ago?"
The direct-to-consumer DNA industry sometimes warns customers of the possibility of unintended consequences. But company involvement stops there.
The two Kincaid brothers declined through a spokesman to talk about their experience, calling it too painful.
Others, with the benefit of genetic distance, are more philosophical.
"I'm sure in the history of the Kinkaide family, there's been some fooling around," said 66-year-old Perry Kinkaide.
"If that's unique to this family, I'd be surprised."
Values and behavior
How many of us are not our fathers' children?
The question has fascinated researchers as a window into the gap between a society's stated values and its behavior. A 2005 analysis of 17 studies -- based on blood and DNA tests of various groups -- concluded that the answer varies depending on country and culture. But the average rate is 4%.
The issue has long lurked in the background of medicine. It's not hard to figure out if your blood type is compatible with Mom's and Dad's(If they are both A positive and you are B positive, you have a problem.)
A recent survey of 56 kidney transplant centers by the University of Maryland showed that 70% had stumbled upon at least one case of non-paternity as a result of testing potential organ donors.
DNA testing has opened the gates of possibility. The potential for surprises exists whenever members of the same family are tested.
For example, researchers looking for the genetic fingerprints of certain diseases have long compared child and parent DNA.
Every so often, mismatches pop up that raise the possibility of hanky-panky.
In research, subjects have signed waivers agreeing that discoveries of non-paternity will not be revealed to them or anybody else.
But in medical practice, the truth has a way of cruelly asserting itself.
In the most common scenario, a child is born with Tay-Sachs disease, cystic fibrosis or another disorder that requires the contribution of a certain gene from each parent. The parents are tested, and the father is found not to carry the gene.
Breaking the news falls to genetic counselors, who often must balance competing ethical imperatives.
"Non-paternity is one of the issues that genetic counselors dread but at some point in their careers will have to deal with," said Andrea Atherton, a counselor at Children's Mercy Hospital in Kansas City, Mo.
Standard practice is to tell only the mother, who usually already suspects it, genetic counselors say.
Ana Morales, a genetic counselor at the University of Miami, recalled the case of a child diagnosed with a type of albinism that can be accompanied by lung and kidney disease.
The mother "told me she was having an affair," Morales said.
"She said she would be in physical danger [if her husband found out]. He had threatened her if she was unfaithful."
Morales did not tell him.
But withholding the information means that the woman's husband lives with the false belief that he is a carrier of a genetic disorder.
That sort of information is far from benign, said Dr. Wayne Grody, a UCLA geneticist. It could convince him to give up on the idea of having children. And in the event that the wife becomes pregnant by her husband, perpetuating the lie could require unnecessary prenatal testing.
"Why would you expose the next fetus to the risk of amniocentesis?" Grody asked.
In the growing world of direct-to-consumer DNA testing, customers are usually on their own to discover and digest non-paternity.
The industry has ballooned to more than three dozen companies from its inception about nine years ago.